This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-136458, filed May 7, 2001; and No. 2001-136459, filed May 7, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for controlling an injection molding machine, and more particularly, to a method for controlling the forward movement speed of a screw in a heating barrel when a molten resin is fed into a mold.
2. Description of the Related Art
In such an injection-molding machine, when a molten resin stored in a heating barrel is fed into a mold by advancing a screw within the heating barrel, the following method has been employed for controlling the speed of the screw.
To explain more specifically, the forward movement speed of the screw is controlled in accordance with a predetermined speed pattern until the screw reaches a predetermined control-mode switching position (hereinafter referred to as a xe2x80x9cdeceleration initiation pointxe2x80x9d). After the screw reaches the predetermined deceleration initiation point, the forward movement speed of the screw is gradually reduced in a feedback manner such that the internal pressure of a hydraulic cylinder for driving the screw reaches a predetermined target pressure (which has been set to correspond to a target internal pressure of the mold). Subsequently, when the screw reaches a predetermined position (called a xe2x80x9cpressure holding initiation pointxe2x80x9d), or when the time passes from the initiation of the forward movement of the screw reaches a predetermined time (called a xe2x80x9cpressure holding initiation timexe2x80x9d), the internal pressure of the hydraulic cylinder is controlled in a feedback manner so as to be maintained at the target pressure.
The step during which the screw reaches the pressure holding initiation point (or the elapsed time from the initiation of forward moving of the screw reaches the pressure holding initiation time) is called a xe2x80x9cfilling stepxe2x80x9d. After the screw reaches the pressure holding initiation point (after the pressure holding initiation time), the step is called a xe2x80x9cpressure holding stepxe2x80x9d.
However, in such a conventional controlling method, the internal pressure of the mold is reduced in the pressure holding step since the molten resin flows within the mold, with the result that the internal pressure of the hydraulic cylinder does not accurately correspond to that of the mold cavity. Therefore, it is difficult to accurately control the internal pressure of the mold, preventing improvement of products in quality and yield.
To overcome such a problem, Japanese Patent No. 2756077 (corresponding to Japanese Publication No. 7186231 A) has proposed a method comprising detecting the internal pressure of the mold in the pressure holding step and setting the internal pressure so as to correspond to the predetermined target value, thereby controlling the forward movement speed of the screw in a feedback manner. In this controlling method, the target internal pressure in the pressure holding step is set at a constant value. Therefore, if mismolding occurs such as burrs, shrinkage cavity or wrinkling, much labor and many steps are required to adjust conditions for molding a good product. Furthermore, the number of types of products is limited whose quality can be improved by this controlling method.
Moreover, in the controlling method for Japanese Patent No. 2756077, the internal pressure of the mold is measured only at a single point. Therefore, if an abnormally excessive pressure is generated at a site other than the pressure detecting point within the cavity, mismolding may sometimes occur around the site.
The present invention has been achieved to overcome the problems associated with a conventional method for controlling the forward movement speed of the screw in an injection-molding machine. A first object of the present invention is to provide a controlling method capable of finding controlling conditions in fewer test operations when mismolding occurs or the production of a new product is initiated. A second object of the present invention is to provide a controlling method for improving products in quality and yield by preventing the pressure of a mold from being generated excessively large.
According to a first aspect of the present invention, there is provided a method for controlling a forward movement speed of a screw in an injection molding machine in which a molten resin stored in a heating barrel is fed into a mold by advancing the screw within the heating barrel, comprising:
controlling the forward movement speed of the screw in accordance with a predetermined speed pattern after the initiation of the forward movement until the screw reaches a predetermined deceleration initiation point;
measuring an internal pressure of the mold and controlling the forward movement speed of the screw in a feedback control manner such that the internal pressure reaches a predetermined pressure, from the deceleration initiation point to a predetermined pressure holding initiation point, said feedback control being performed using a first set of PID (proportional-plus-integral-plus-derivative) coefficients; and
measuring the internal pressure of the mold and controlling the forward movement speed of the screw in a feedback control manner such that the internal pressure of the mold changes in accordance with a predetermined pattern of target pressure versus time, after the screw reaches the pressure holding initiation point, said feedback control being performed using a second set of PID coefficients.
According to the method for controlling the forward movement speed of the present invention, the target pressure of the mold in the pressure holding step (which is performed after the screw reaches the pressure holding initiation point) is not necessary to correspond to that of the deceleration step (performed from the deceleration initiation point to the pressure holding initiation point). The target internal pressure of the mold during the pressure holding step can be set at a more appropriate value or controlled in accordance with more appropriate pattern, compared to a conventional method. As a result, it is possible to determine appropriate control conditions in fewer trial operations than in the conventional controlling method. Furthermore, a larger number of types of molded products can be formed by the controlling method of the present invention compared to conventional controlling methods.
Furthermore, the set of PID coefficients in the PID operation expression within a feedback loop are replaced by different set of PID coefficients when the screw reaches the pressure holding initiation point. Thereby, it is possible to change the internal pressure of the mold in the form of a smooth curve.
In the case where the internal pressure of the mold is changed in accordance with the program, for example, the target pressure is switched stepwise as time passes after the screw reaches the pressure holding initiation point.
In this case, it is preferable that the integral term of a PID operation expression within a feedback control loop be cleared and replaced by zero when the target pressure is switched stepwise as time passes.
As a modification of the controlling method, the pressure holding step may be initiated when the time elapsed from the initiation of the forward movement of the screw reaches a predetermined time (called a pressure holding initiation time).
According to a second aspect of the present invention, there is provided a method for controlling a forward movement speed of a screw in an injection molding machine in which a molten resin stored in a heating barrel is fed into a mold by advancing the screw within the heating barrel, comprising:
controlling the forward movement speed of the screw in accordance with a predetermined speed pattern after the initiation of the forward movement until the screw reaches a predetermined deceleration initiation point;
measuring an internal pressure of the mold at a plurality of points within the mold and controlling the forward movement speed of the screw in a feedback control manner such that the highest of internal pressures thus measured reaches a first target pressure, from the deceleration initiation point to a predetermined pressure holding initiation point, said feedback control being performed using a first set of PID coefficients; and
measuring the internal pressure of the mold at the plurality of points within the mold and controlling the forward movement speed of the screw in a feedback control manner such that the highest internal pressure is maintained at a second target pressure, after the screw reaches the pressure holding initiation point, said feedback control being performed using a second set of PID coefficients.
According to the method for controlling the forward movement speed of a screw of the present invention, the forward movement speed of the screw is controlled in such a manner that the highest of internal pressures (of those measured at a plurality of points within the mold) corresponds to the target pressure after the screw reaches the deceleration initiation point. It is therefore possible to prevent generation of an excessive large pressure within the mold.
Furthermore, the set of PID coefficients in the PID operation expression within the feedback loop are replaced by different set of PID coefficients when the screw reaches the pressure holding initiation point. Thereby, it is possible to change the internal pressure of the mold in the form of a smooth curve.
It is preferable that, after the screw reaches the pressure holding initiation point, the forward movement speed of the screw is controlled using a second set of coefficients and in accordance with a program such that the highest internal pressure changes in accordance with a predetermined pattern of target pressure versus time.
As a modification of the controlling method mentioned above, the pressure holding step may be initiated when the time elapsed from the initiation of the forward movement of the screw reaches a predetermined time (called a pressure holding initiation time).
The method for controlling the forward movement speed of a screw according to the present invention can be applied to not only a hydraulic injection-molding machine in which the screw is driven by a hydraulic cylinder but also an electric injection-molding machine in which the screw is driven by an electric motor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.